1. Field of the Invention
The present invention relates to a method for manufacturing a sintered body of silicon nitride by sintering a compact of silicon nitride.
2. Description of the Prior arts
Silicon nitride is widely known as material for sintering, but silicon nitride alone is hard to sinter. Powders of Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, MgO, La.sub.2 O.sub.3, AlN, BeSiN.sub.2 and BeAl.sub.2 O.sub.4 are added, as a sintering assistant, to silicon nitride powders to prepare powder material. From this powder material a compact of a complicated shape is comparatively easy to be formed by a method of slip casting.
In the method of slip casting, the powder material is dispersed into a liquefied dispersion medium to be formed into slurry and this slurry is cast into a mold having a desired cavity shape. Depending on methods of solidifying slurry cast, the method for slip casting is classified into the following two methods:
(a) a mold having a feature by absorbing liquid which is made of gypsum, porous resin or the like is used. This mold absorbs a part of water content in the slurry to form a feature of maintaining the shape of a compact once molded. And then, the mold is demolded to obtain a compact of a desired shape. In this method, water and alcohol are used as a dispersion medium to disperse the powder material.
(b) The slurry material is cast in a mold which is cooled down upto the melting point temperature of a dispersion medium or less, or the mold is cooled upto the same temperature after the powder material is cast therein to solidify the dispersion medium, thereby maintaining the shape of a molded compact. And then, the mold is demolded to obtain a compact of the desired shape. In this method, paraffin is used as the dispersion medium.
The compact thus prepared is sent to the process wherein the dispersion medium is removed. This removal of the dispersion medium is carried out through air drying or vaporization by heating when the dispersion medium is water and alcohol, and through the vaporization and thermal decomposition of paraffin by means of heating it at 500.degree. C. when it is used as the dispersion of medium.
The compact from which the dispersion medium has been removed is transferred to the process of sintering, and the compact is buried into powdery fillers filled in a crucible in advance of the sintering.
This is because of reducing decomposition of silicon nitride and avoiding direct contact of the compact with the graphite crucible which impairs sintering. The powdery fillers are those which Si.sub.3 N.sub.4, SiO.sub.2, Al.sub.2 O.sub.3, AlN, MgO and BN are appropriately mixed into. Sintering operation is applied to the compact covered with the powdery fillers The operation is carried out at 1,600.degree. to 2,000.degree. C. and in the atmosphere of nitrogen having the pressure or more at which silicon nitride is decomposed so as to prevent the thermal decomposition thereof The pressure of the nitrogen atmosphere is required to be raised in proportion to the rise of the temperature, but within the aforementioned range of the temperature, 0 to 10kg/cm.sup.2 .multidot.G is ordinarily adopted. In addition, it is known that the dispersion medium remaining in the compact after the removal is perfectly removed by degassing, for example, at 10.sup.-2 Torr and 1,000.degree. C.
On the other hand, there is a method wherein a density of silicon nitride compact is increased by hot isostatic pressing process Namely, the process is applied to the compact at 1,600.degree. to 1,900.degree. C. and in the atmosphere of the gas such as N.sub.2 and Ar having 1,000 to 1,300 kg/cm.sup.2 .multidot.G. In this method, a film layer impermeable to gas is formed on the surface of the compact and heat and pressure is applied thereto from the outside of the film layer. Those methods are disclosed, for example, in a Japanese Examined Patent Publication No. 35870/84 and a West German Patent No. 3,403,917.
Said Japanese Examined Patent Publication mentions a method wherein:
(a) a pre-compact is dipped in slurry of powder material for forming a film layer thereof;
(b) the film layer formed by said dipping is dried;
(c) the step of said dipping and drying is repeatedly carried out, and through this repetition, an inner porous film layer comprising a high melting glass forming substance or a high melting metal forming substance, and an outer porous film layer comprising a low melting glass forming substance or a low melting metal forming substance are formed; and
(d) the outer porous film layer is made to be impermeable by means of heating, and subsequently the inner porous film layer to be impermeable.
Said West German Patent discloses another method repeating the step of said dipping and drying to form porous film layers as shown in said Japanese Examined Patent Publication. But in this method, as an inner porous film layer, a substance having no sintering assistant, and as an outer porous film layer, a substance having a sintering assistant, respectively are used. By means of heating, sintering of the outer film layer is promoted thereby to change it into a fine impermeable one.
In case of silicon nitride, pressure and temperature to which hot isostatic pressing treatment contributes is at least 100 kg/cm.sup.2 .multidot.G and 1,600.degree. C., although it depends on a feature of a sintering assistant. The above-mentioned method using a film layer impermeable to gas, as a pressure medium, is applicable at the critical pressure or more and the critical temperature or more.
Those prior art methods of sintering silicon nitride have the following disadvantages:
(a) when sintering silicon nitride is carried out without powdery fillers, decomposition of silicon nitride is unavoidable;
(b) even if the sintering is done with powdery fillers wherein the pre-compact is buried, the perfect exclusion of decomposition of silicon nitride cannot be attained, and a layer of deterioration in quality is formed on the surface of the compact; and
(c) because the heat conduction of the powder fillers is very bad, it takes a long time for the sintering when they are used.
In addition, the prior art process of forming an impermeable film layer comprises many steps such as removing a dispersion medium from a compact, coating slurry of a porous film layer forming substance, and drying, and the pre-process for sintering, thus, becomes intricate.